The distribution and morphology of GnRH neurons was examined in the forebrains of five ovariectomized (OVX) and five OVX/estradiol-treated rhesus macaques. The brains were perfusion fixed with 4% paraformaldehyde and frozen sections (25 fm) were processed for immunocytochemistry (ICC) using a monoclonal GnRH antibody. One quarter of the sections from each brain were examined and, on average, 634 immunopositive neurons were detected. They were widely distributed throughout the forebrain but two major populations predominated, one in the preoptic-medial septal area and one in the medial basal hypothalamus. The morphology of these neurons was generally fusiform with either a prominent single or multiple neurites (ratio 3:1). However, approximately one third of the immunopositive neurons had a markedly different morphology. They were located more laterally and were characterized by a more spherical perikaryon with very short neurites. To investigate whether these neurons were indeed GnRH producing cells rather than GnRH target cells, immunostained sections were processed for in situ hybridization using a monkey GnRH cRNA probe. The lateral immunopositive neurons showed clear-cut GnRH mRNA expression and, interestingly, also showed a high level of mRNA expression for the AMPA-preferred glutamate receptor subunit, GluR1. As expected, the major fusiform populations of GnRH neurons also showed a high level of GnRH mRNA expression but, surprisingly, had no detectable level of GluR1 mRNA. Although no differences were found between the GnRH neurons of OVX and OVX/estradiol-treated animals, these data demonstrate that GnRH neurons of higher primates exist in distinct populations, which differ not only in morphology but most likely also in function. (Findings from this study were presented in 1996 at the International Congress of Endocrinology, held in San Francisco, CA).